Cyclopentadiene derivatives for carbon-carbon composites

ABSTRACT

A thermosetting resin is formed from cyclopentadiene and a propargyl halide The resin can be used with carbon fibers to form a carbon—carbon composite.

This is a divisional of application Ser. No. 08/835,893 filed Apr. 8,1997, now U.S. Pat. No. 5,973,092. This application also claims benefitof Provisional U.S. application Ser. No. 60/614,927 filed Apr. 8, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to carbon—carbon composites and monomersused in their formation. More particularly, the invention relates to anew family of monomers used in the formation of carbon—carboncomposites.

Interest in carbon—carbon composites has grown exponentially in recentyears. The combination of high specific strength retention at elevatedtemperatures and chemical inertness have led to their use inapplications such as aircraft brakes, space vehicle heatshields androcket nozzles.

Although the properties of carbon—carbon composites make them veryuseful, there are inherent problems with typical precursors such aspitch and many thermoset resins used to fabricate these composites. Oneof the most severe limitations of the presently used precursors is lowchar yields during pyrolysis. Typical thermoset precursors yield 50-65weight percent carbon after pyrolysis. This results in the need forrepeated impregnation cycles with resin to achieve a void-free sample.

Another problem with typical thermoset and pitch based carbon precursorsarises from the high viscosities and intractable nature of thesematerials. This often results in poor wetting of the reinforcing fiberand/or the need for higher pressures and temperatures during theimpregnation cycles.

Accordingly, it would be a significant advancement in the art to providea precursor for carbon—carbon composites which had a low viscosity. Itwould be a further advancement in the art if such a resin had a charyield of greater than about 75%.

SUMMARY OF THE INVENTION

One aspect of the present invention comprises a new family of monomersbased on alkene and alkyne derivatives of cyclopentadiene useful in theformation of carbon—carbon composites. These monomers are synthesized byreacting freshly prepared cyclopentadiene with allyl halides orpropargyl halides in the presence of excess aqueous base and anappropriate phase transfer catalyst. Suitable catalysts include crownethers, polyoxyethylene and quaternary ammonium salts. Purification ofthe resulting monomers involves washing with aqueous acid and base, andin some cases, extraction distillation or chromatography.

In another aspect of the present invention, the cyclopentadienederivative resins are used as a thermoset precursor for carbon—carboncomposites. Carbon fibers are coated with the resin and thermally cured.In one preferred embodiment, carbon fibers were coated with apropargylated cyclopentadiene (PCP) resin in a stainless steel mold.Pressure was applied to squeeze out excess resin and consolidate thefibers. The resin was then cured at 165° C. for 6 hours and 250° C. for6 hours.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a new family of thermosettingresins and their use in the formation of carbon—carbon composites. Theresins comprise substituted cyclopentadiene.

In a preferred embodiment, the resins are synthesized by reactingfreshly prepared cyclopentadiene with propargyl halides in the presenceof excess aqueous base and an appropriate phase transfer catalyst.Suitable catalysts include crown ethers, polyoxyethylene and quaternaryammonium salts. One preferred catalyst is tetrabutylammonium bromide.Suitable propargyl halides include propargyl chloride and propargylbromide.

These new monomers are highly reactive and oxidatively unstable.Exposure to air leads to incorporation of alcohol, acid and aldehyde orketone functionality. Accordingly, during preparation it is important toremove and exclude air from the reaction to prevent oxidation of theresin.

In one embodiment, propargyl chloride is reacted with cyclopentadienewith an excess aqueous base to form the compounds set for in equation 1wherein n equals 2-6.

In a second embodiment propargyl bromide is reacted with cyclopentadienein the presence of excess aqueous base and a suitable catalyst to formthe reaction product set forth in Equation 1 where n=2-6.

After synthesis, the resins are extracted and purified. In a preferredembodiment, the resins are extracted with toluene and washed with diluteacid, water and brine to remove the phase transfer catalyst. Yields ofresin are nearly quantitative with respect to the cyclopentadiene. Theproduct consists of a mixture of isomers of multiply substitutedcyclopentadienes with degrees of functionality ranging from 2 to 6propargyl groups per ring.

Carbon-carbon composite formation involves simple impregnation of carbonfiber preforms or mats with the liquid resin followed by thermal cure.The thermal cure is generally carried out in the absence of air whichcan cause side reactions such as the incorporation of alcohol, acid andaldehyde or ketone functionality. The composites are cured attemperatures up to about 350° C.

In one preferred embodiment, the thermal cure is performed at 165° C.for 6 hours and 250° C. for 6 hours. The cure times and temperatures canbe adjusted as necessary depending upon the size of the samples andtheir intended use.

In another preferred embodiment, initiators or catalysts can be used inthe curing process including free radical initiators, cationicinitiators and metal salts capable of catalyzing polyadditon andmethathesis reactions. Such compounds are known to those skilled in theart.

Once cured, the composite products are thermally and oxidatively stableto high temperatures; e.g., heating to 1000° C. in the absence of aircauses less than 25% weight loss for these composites. This is extremelylow in comparison to other resins currently used for carbon—carboncomposite formation.

The advantages of the monomers of the present invention are their lowviscosity for ease if impregnation and handling, their facile curethrough uncatalyzed thermal reaction or initiated/catalyzed reactions togive highly condensed carbon networks, and extremely good thermal andoxidative stability of cured materials.

The composites of the present invention have many applications includingthermally conductive brake linings for automotive and aircraft brakes,high thermal stability engine parts (piston and cylinder walls) andaerospace components in both structural and non-structural applications.

EXAMPLE 1

Dicyclopentadiene was cracked thermally at 175° C. to obtaincyclopentadiene. This cyclopentadiene was then mixed neat with aconcentrated NaOH solution in the presence of tetrabutylammonium bromidephase transfer catalyst. Propargyl bromide was added to the reactiondropwise with rapid stirring and cooling as needed. Air was removed andexcluded from the reaction vessel in order to prevent oxidation of theresin.

After essentially all of the cyclopentadiene was reacted the product wasextracted with toluene and washed with dilute acid, water and brine toremove the phase transfer catalyst. The product consisted of a mixtureof isomers of multiply substituted cyclopentadienes with degrees offunctionality ranging from 2 to 6 propargyl groups per ring.

A sample of this PCP resin cured at 250° C. was pyrolized at 1000° C.The resin maintained approximately 75% of the its initial weight.

EXAMPLE 2

A carbon—carbon composite was fabricated using the resin from Example 1.AS-4 unsized carbon fibers manufactured by Hercules Inc. were cut tolength and placed unidirectionally in a stainless steel mold. Thesefibers were then coated with the PCP resin of Example 1. The process wasrepeated until a part of appropriate thickness was formed within themold. A stainless steel plug was then placed over the sample andmechanical pressure was applied to squeeze out excess resin andconsolidate the fibers.

The sample was then cured at 165° C. for 6 hours and 250° C. for 6hours. The resulting composite was 70±3 volume percent carbon fiber. Thecured resin had a glass like appearance. During acoustic testing, thecomposite had a metallic ring, characteristic of a void-free sample.

FTIR was used to characterize the thermal cure of the PCP resin. Thepeak at 3290 cm³¹ ¹ representative of the C—H stretch of the propargylgroup, and the peak at 2120 cm³¹ ¹ representative of the triple bondstretching of the propargyl functionality, were completely gone in thecured sample indicating complete reaction of the functional groups. Theappearance of a broad peak in the 3400-3500 cm³¹ ¹ region as well as the1700-1600 cm¹ region (OH and carbonyl groups) suggest that oxidativecross-linking may have been responsible for a significant amount of thecure reaction.

As can be seen from the foregoing, the present invention provides anovel thermoset resin which can be used in carbon—carbon composites. Thecomposites exhibit good consolidation, matrix-fiber adhesion andmechanical properties.

While the invention has been described with respect to the presentlypreferred embodiments, it will be appreciated by those skilled in theart that changes and modifications can be made without departing fromthe spirit or scope of the invention. Accordingly, the scope of theinvention is defined by the appended claims rather than by the foregoingdescription and all changes or modifications which come within themeaning and range of the claims are to be embraced within their scope.

We claim:
 1. A carbon—carbon composite comprising carbon fibers and a cured alkyne derivative of cyclopentadiene, said cured alkyne derivative having a char yield at 1,000° C. of about 75%.
 2. A carbon—carbon composite as defined in claim 1 wherein the cyclopentadiene derivative comprises the reaction product of cyclopentadiene with a propargyl halide.
 3. A carbon—carbon composite as defined in claim 2 wherein the propargyl halide comprises propargyl bromide.
 4. A carbon—carbon composite as defined in claim 2 wherein the propargyl halide comprises propargyl chloride.
 5. A carbon—carbon composite as defined in claim 2 wherein the cyclopentadiene derivative is thermally cured.
 6. A carbon—carbon composite comprising carbon fibers and a cured alkene derivative of cyclopentadiene, said cured alkene derivative having a char yield at 1,000° C. of about 75%. 